Knights of the Nine is an official add-on for Oblivion, revolving around the titular order as they recover the lost relics of the Divine Crusader. A list of frequently-asked questions and their answers can be found on the FAQs page.

A fallen King has been unchained from the darkness of Oblivion to seek vengeance upon the Gods who banished him. Only a champion pure of heart can vanquish the evil that has been released upon the land. You must heed the call, reclaim the lost relics of the Divine Crusader, and return the Nine to glory. New dungeons, characters, quests, and mysteries await. —Official Summary

Completing the Knights of the Nine questline will require you to abolish your Infamy.

The variety of items, powers, and services rewarded from completing the quests may be particularly useful for new characters.

You can only equip the Relics gained in the quests if your infamy is 1 or less. If your infamy ever reaches 2 or more, you must repeat the Pilgrimage to a Wayshrine of every deity. Items and effects that temporarily increase your Infamy, such as the Gray Cowl of Nocturnal will make you unable to equip the Relics until you unequip the items or the effects wear off.

Unlike other official plug-ins, Knights of the Nine introduces new dialogue into NPC interactions throughout the entire game. For instance, after several of the Crusader's Relics are claimed, NPCs will mention them in rumors and in conversations with other NPCs. Similarly, by completing the questline, NPCs will sometimes refer to you as a Holy Knight or Divine Crusader. If your infamy rises above 0 after completing the questline, NPCs will sometimes greet you saying: "I used to think you were a hero. A holy knight and all that. I guess I should have known better." or "I used to admire you. But it turns out you're just a sinner like all the rest of us."